Custom result builder declaration in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code using a custom result builder?

Consider the following Swift code that uses a custom result builder named StringListBuilder. What will be printed when print(buildList()) is called?

Swift

import Foundation

@resultBuilder
struct StringListBuilder {
    static func buildBlock(_ components: String...) -> [String] {
        return components
    }
}

func buildList() -> [String] {
    StringListBuilder.buildBlock(
        "apple",
        "banana",
        "cherry"
    )
}

print(buildList())

A ["apple", "banana", "cherry"]

B "applebanana cherry"

C ["apple banana cherry"]

D SyntaxError

💡 Hint

Look at how the buildBlock method collects its string components into an array.

The buildBlock method takes a variable number of String arguments and returns them as an array. So the function buildList() returns an array of strings, which is printed as ["apple", "banana", "cherry"].

❓ Predict Output

intermediate

What error does this Swift custom result builder code produce?

Given this Swift code with a custom result builder, what error will occur when compiling?

Swift

@resultBuilder
struct IntBuilder {
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }
}

func buildSum() -> Int {
    IntBuilder.buildBlock(
        1,
        2,
        "3"
    )
}

print(buildSum())

A Output: 6

B RuntimeError: Cannot add String to Int

C TypeError: Cannot convert value of type 'String' to expected argument type 'Int'

D SyntaxError: Missing colon

💡 Hint

Check the types of the arguments passed to buildBlock.

The buildBlock expects only Int arguments, but a String "3" is passed, causing a compile-time type error.

🧠 Conceptual

advanced

Which option correctly implements a custom result builder that supports optional components?

You want to create a custom result builder that can handle optional strings, including them only if they are non-nil. Which implementation of buildOptional is correct?

A

static func buildOptional(_ component: String?) -> [String] {
    if component != nil { return [component!] } else { return [""] }
}

B

static func buildOptional(_ component: String?) -> [String] {
    component.map { [$0] } ?? []
}

C

static func buildOptional(_ component: String?) -> String {
    component ?? ""
}

D

static func buildOptional(_ component: String?) -> [String] {
    [component!]
}

💡 Hint

Think about safely unwrapping optionals and returning an empty array if nil.

Option B safely unwraps the optional and returns an array with the string if non-nil, or an empty array if nil. This matches the expected behavior for buildOptional in a result builder that produces arrays.

❓ Predict Output

advanced

What is the output of this Swift custom result builder code?

Given this Swift code with a custom result builder, what will be printed when print(buildNumber(condition: true)) is called?

Swift

@resultBuilder
struct NumberBuilder {
    static func buildBlock(_ components: Int...) -> Int {
        components.reduce(0, +)
    }

    static func buildEither(first component: Int) -> Int {
        component
    }

    static func buildEither(second component: Int) -> Int {
        component
    }
}

func buildNumber(condition: Bool) -> Int {
    NumberBuilder.buildBlock(
        1,
        condition ? NumberBuilder.buildEither(first: 2) : NumberBuilder.buildEither(second: 3),
        4
    )
}

print(buildNumber(condition: true))

A Error: Missing buildOptional method

B Output: 10

C Error: Cannot convert value of type 'Int' to expected argument type '[Int]'

D Output: 7

💡 Hint

The ternary selects the first branch (2) when condition is true. Sum 1 + 2 + 4.

The ternary expression selects NumberBuilder.buildEither(first: 2), returning 2. buildBlock(1, 2, 4) sums to 7 via reduce(0, +).

🚀 Application

expert

How many items are in the resulting array from this custom result builder usage?

Given this Swift code with a custom result builder that supports conditionals and optionals, how many strings are in the array returned by buildMenu(showDessert: true)?

Swift

@resultBuilder
struct MenuBuilder {
    static func buildBlock(_ components: [String]...) -> [String] {
        components.flatMap { $0 }
    }

    static func buildOptional(_ component: [String]?) -> [String] {
        component ?? []
    }

    static func buildEither(first component: [String]) -> [String] {
        component
    }

    static func buildEither(second component: [String]) -> [String] {
        component
    }
}

func buildMenu(showDessert: Bool) -> [String] {
    MenuBuilder.buildBlock(
        ["Salad", "Soup"],
        showDessert ? MenuBuilder.buildEither(first: ["Cake"]) : MenuBuilder.buildEither(second: []),
        MenuBuilder.buildOptional(nil)
    )
}

let menu = buildMenu(showDessert: true)
print(menu.count)

A 3

B 2

C 4

D 1

💡 Hint

Count all strings included by the builder, including conditionals and optionals.

The builder combines ["Salad", "Soup"] plus ["Cake"] because showDessert is true, and buildOptional(nil) returns []. So total items are 3.

Practice

1. What is the main purpose of declaring a @resultBuilder in Swift?

easy

A. To handle errors automatically in functions

B. To create a new data type like a class or struct

C. To optimize memory usage during runtime

D. To define how multiple code parts combine into a single result

Solution

1. Step 1: Understand the role of @resultBuilder

   A @resultBuilder is used to customize how multiple code blocks combine into one result, making code cleaner.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated concepts like error handling, data types, and optimization, which are not the purpose of result builders.

3. Final Answer:

   To define how multiple code parts combine into a single result -> Option D

4. Quick Check:

   Result builder = combine code parts [OK]

Hint: Result builders combine code parts into one result [OK]

Common Mistakes:

• Confusing result builders with classes or structs
• Thinking result builders handle errors
• Assuming result builders optimize memory

2. Which of the following is the correct way to start declaring a custom result builder in Swift?

easy

A. @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } }

B. @resultBuilder class MyBuilder { func buildBlock() {} }

C. class MyBuilder { func buildBlock() -> Int { 0 } }

D. struct MyBuilder { static func buildBlock() {} }

Solution

1. Step 1: Identify correct syntax for result builder

   A custom result builder must be declared with @resultBuilder attribute on a struct, and have static methods like buildBlock returning a value.

2. Step 2: Check each option

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } correctly uses @resultBuilder, declares a struct, and has a static buildBlock method returning a String. The other options miss the attribute, use class instead of struct, or have non-static buildBlock.

3. Final Answer:

   @resultBuilder struct MyBuilder { static func buildBlock() -> String { "" } } -> Option A

4. Quick Check:

   Result builder = @resultBuilder + struct + static methods [OK]

Hint: Use @resultBuilder on a struct with static buildBlock method [OK]

Common Mistakes:

• Using class instead of struct for result builder
• Forgetting the @resultBuilder attribute
• Making buildBlock non-static

3. Given this custom result builder and usage, what is the output?

 @resultBuilder
struct IntSumBuilder {
    static func buildBlock(_ numbers: Int...) -> Int {
        numbers.reduce(0, +)
    }
}

func sum(@IntSumBuilder _ content: () -> Int) -> Int {
    content()
}

let result = sum {
    5
    10
    3
}
print(result)

medium

A. 5103

B. 18

C. Error: buildBlock requires array

D. 0

Solution

1. Step 1: Understand buildBlock behavior

   The buildBlock method takes multiple Int values and sums them using reduce(0, +).

2. Step 2: Evaluate the sum call

   The closure passed to sum returns three Int values: 5, 10, and 3. The builder sums them: 5 + 10 + 3 = 18.

3. Final Answer:

   18 -> Option B

4. Quick Check:

   Sum of 5, 10, 3 = 18 [OK]

Hint: buildBlock sums all Int inputs passed in closure [OK]

Common Mistakes:

• Thinking values concatenate as strings
• Expecting buildBlock to receive an array instead of variadic
• Confusing output with zero or error

4. Identify the error in this custom result builder declaration:

 @resultBuilder
struct TextBuilder {
    func buildBlock(_ components: String...) -> String {
        components.joined(separator: ", ")
    }
}

medium

A. buildBlock must be static

B. buildBlock must return Int

C. struct cannot be used with @resultBuilder

D. buildBlock cannot take variadic parameters

Solution

1. Step 1: Check method requirements in result builder

   Methods like buildBlock inside a @resultBuilder must be declared as static so they can be called without an instance.

2. Step 2: Analyze the given code

   The buildBlock method is missing the static keyword, which causes a compile error.

3. Final Answer:

   buildBlock must be static -> Option A

4. Quick Check:

   buildBlock method = static [OK]

Hint: buildBlock must always be static in result builders [OK]

Common Mistakes:

• Omitting static keyword on buildBlock
• Thinking buildBlock must return Int always
• Believing struct can't be used with @resultBuilder

5. You want to create a custom result builder that builds a dictionary from key-value pairs written inside a closure. Which of the following buildBlock implementations correctly combines multiple (String, Int) tuples into a single dictionary?

hard

A. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } }

B. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } }

C. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) }

D. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs }

Solution

1. Step 1: Understand the goal

   The builder should combine multiple key-value pairs into a single dictionary.

2. Step 2: Analyze each option

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } uses Dictionary(uniqueKeysWithValues:) which correctly creates a dictionary from tuples. static func buildBlock(_ pairs: [(String, Int)]) -> [String: Int] { pairs.reduce([:]) { $0 + $1 } } tries to reduce with + which is invalid for dictionaries. static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { pairs.map { [$0.0: $0.1] } } maps to an array of dictionaries, not a single dictionary. static func buildBlock(_ pairs: (String, Int)...) -> [(String, Int)] { pairs } returns an array of tuples, not a dictionary.

3. Final Answer:

   static func buildBlock(_ pairs: (String, Int)...) -> [String: Int] { Dictionary(uniqueKeysWithValues: pairs) } -> Option C

4. Quick Check:

   Use Dictionary(uniqueKeysWithValues:) to combine tuples [OK]

Hint: Use Dictionary(uniqueKeysWithValues:) to build dictionary from tuples [OK]

Common Mistakes:

• Returning array of dictionaries instead of one dictionary
• Using invalid + operator to combine dictionaries
• Returning array of tuples instead of dictionary